Refactor for TDD workflow with Vitest and ESLint

- Move API keys from hardcoded values to .env via dotenv
- Extract business logic into src/trading.ts with dependency injection
- Add typed AlpacaClient interface, replace `any` on Alpaca class
- Add Vitest test suites for trading logic and Alpaca wrapper (14 tests)
- Set up ESLint with @typescript-eslint for linting
- Fix getAsset to pass symbol string directly to SDK
- Fix typo (alpacha -> alpaca), remove unused ws/node-fetch deps
- Update .gitignore for node_modules and .env

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

eslint.config.js

@@ -0,0 +1,20 @@
+const tseslint = require('@typescript-eslint/eslint-plugin');
+const tsparser = require('@typescript-eslint/parser');
+
+module.exports = [
+    {
+        files: ['src/**/*.ts'],
+        languageOptions: {
+            parser: tsparser,
+            ecmaVersion: 'latest',
+            sourceType: 'module',
+        },
+        plugins: {
+            '@typescript-eslint': tseslint,
+        },
+        rules: {
+            ...tseslint.configs.recommended.rules,
+            'no-console': 'off',
+        },
+    },
+];